1. Field of the Invention
The present invention relates generally to a burst mapping method and apparatus in a mobile communication system, and, in particular, to a method and apparatus for efficiently performing burst mapping using 16-ary Quadrature Amplitude Modulation (16-QAM) and symbol mapping in a Global System for Mobile communication (GSM)/Enhanced Data rates for GSM Evolution (EDGE) system.
2. Description of the Related Art
In the GSM/EDGE system, which is a European communication system, different coding schemes can be used according to the modulation scheme and the coding scheme.
The modulation scheme used in the GSM/EDGE system can include Gaussian Minimum Shift Keying (GMSK) and 8-ary Phase Shift Keying (PSK) modulation schemes. The GMSK scheme, which is a scheme for limiting a bandwidth by passing binary data through a Gaussian low-pass filter and then performing frequency modulation with a predetermined deviation ratio, allows an interval between two frequencies to continuously change, thereby providing high spectrum concentration and high out-band spectrum suppression. The 8-PSK scheme, which is a scheme for mapping data to a phase-shifted code of a sub-carrier for modulation, can increase frequency efficiency.
Thirteen techniques for Packet Data Traffic Channels (PDTCH) are defined as a coding scheme used in the GSM/EDGE system. The thirteen techniques include four schemes of Coding Schemes (CSs), CS-1/2/3/4 for General Packet Radio Service (GPRS), and nine schemes of Modulation and Coding Schemes (MCSs), MCS-1 to MCS-9 for Enhanced General Packet Radio Service (EGPRS).
During actual communication, one of various combinations of the modulation schemes and the coding techniques is selected and used. The combinations can be identified as MCSs.
EDGE is an extension scheme for a GSM data service, and the EDGE system generally supports Enhanced Circuit-Switched Data (ECSD) and EGPRS. EGPRS uses nine schemes of MCS-1 to MCS-9, each combined of a modulation scheme and a coding scheme. MCS-1 to MCS-4 each use the GMSK modulation scheme, and MCS-5 to MCS-9 each use the 8-PSK modulation scheme. One of the nine MCSs can be determined according to input data.
With reference to the accompanying drawings, a detailed description will be made of a structure of a transceiver for processing input data, for individual MCS types.
FIG. 1 illustrates a structure of a downlink transmitter using MCS-5 to MCS-9 in an EDGE system. Referring to FIG. 1, a Radio Link Control (RLC) packet data block (RLC block) 110 separately outputs Uplink State Flag (USF), header and user data to an encoder 120 according to input user data. Generally, one RLC packet is composed of USF, header and source data.
The encoder 120 can include a USF precoding block 121 for USF coding; a cyclic code adder 123 and a convolutional encoder 125 for header coding; and a cyclic code adder 127 and a convolutional encoder 129 for data coding. The USF, header and data received from the RLC block 110 are input to the USF precoding block 121, cyclic code adder 123 and cyclic code adder 127, respectively, and then coded therein. That is, the USF undergoes precoding in the USF precoding block 121, and the header undergoes convolutional coding in the convolutional encoder 125 after a Cyclic Redundancy Check (CRC) code is added thereto in the cyclic code adder 123. In this manner, the header undergoes coding. Similar to the header, the data undergoes coding through the cyclic code adder 127 and the convolutional encoder 129.
An interleaver 130 for the header interleaves the coded header output from the convolutional encoder 125.
An interleaver 140 for the data interleaves the coded data output from the convolutional encoder 129.
A burst mapper 150 performs burst mapping on the coded bits of the USF, header and data output, respectively, from the USF precoding block 121 and the interleavers 130 and 140, to allocate the coded bits to four bursts in a distributed manner. The burst mapper 150 outputs the bits allocated to four bursts, to an 8-PSK modulator 160.
The 8-PSK modulator 160 performs 8-PSK modulation on the bursts received from the burst mapper 150 before transmission.
A detailed description of a device additionally needed for transmitting the modulated signal, for example, a Digital-to-Analog (D/A) converter is known to those of skill in the art and therefore is omitted herein.
FIG. 2 illustrates a structure of a downlink receiver using MCS-5 to MCS-9 in an EDGE system. Referring to FIG. 2, an equalizer/demodulator 210 performs signal-to-noise equalization on the signal received from the transmitter of FIG. 1, performs demodulation thereon, and then outputs the resulting signal to a burst demapper 220.
The burst demapper 220 demaps the input signal into the bits before they were mapped/allocated to four bursts in the transmitter, i.e. into signaling data such as user data (data source), USF, header, etc. The signaling data means control information.
A deinterleaver 230 performs deinterleaving on the bits output from the burst demapper 220.
A channel decoder 240 performs channel decoding on the bits output from the deinterleaver 230. In this manner, the original data is restored.
Although the receiver can further include a Radio Frequency (RF) unit for receiving an analog signal over the air, and an Analog-to-Digital (A/D) converter for converting the analog signal into a digital signal, the function and structure of the RF unit and A/D converter is known to those of skill in the art and therefore a description thereof will be omitted herein.
However, the existing MCS-5 to MCS-9 using the 8-PSK modulation scheme, although they can increase frequency efficiency due to a characteristic of the 8-PSK scheme, are apt to suffer from noise because of a decrease in the distance between phase states.
The 3rd Generation Partnership Project (3GPP) in charge of standardization for the GSM/EDGE system, especially the GSM/EDGE Radio Access Network (GERAN) system, is conducting a discussion on GERAN Evolution. In the ongoing discussion, several methods for system performance improvement are proposed, and one of the proposed methods is to introduce the turbo code and 16-QAM modulation scheme used in Universal Mobile Telecommunication Service (UMTS) to the existing GERAN system. The currently considered turbo code, having a mother coding rate of ⅓, is the same turbo code as that in the existing UMTS system. With use of the turbo code, one information bit frame can be divided into information bits and parity bits through coding. In other words, when one information bit frame is input to a turbo encoder, the turbo encoder separates the one information bit frame into information bits (or systematic bits) and parity bits. That is, the coded output is separated into an information bit stream and a parity bit stream after undergoing a puncturing and rate matching algorithm according to a channel coding rate.
Recently, there has been developed a technology for mapping information bits and parity bits according to priority by disposing two interleavers at an output stage of the turbo encoder and using characteristics of the turbo encoder. This technology is called Symbol Mapping based on Priority (SMP). SMP interleaves information bits and parity bits independently, and then maps the interleaved information bits and parity bits to higher-reliability bit positions and lower-reliability bit positions according to priority of the bits, before transmission. In other words, important data is allocated to the higher-reliability bit positions in a modulation symbol, and less important data is allocated to the lower-reliability bit positions in the modulation symbol before being transmitted, thereby increasing reliability of the system.
With reference to the accompanying figures, a detailed description will now be made of a 16-QAM scheme, another proposed scheme of the GERAN Evolution system.
FIG. 3 illustrates a Gray-encoded signal constellation available in a 16-QAM modulation scheme of a UMTS system. Referring to FIG. 3, four signal points are located in each area in a quadrant composed of an In-Phase (I) axis and a Quadrature-Phase (Q) axis. That is, S1, S2, S3, S4 are located in an area one (A1) 310; S5, S6, S7, S8 are located in an area two (A2) 320; S9, S10, S11, S12 are located in an area three (A3) 330; and S13, S14S15, S16 are located in an area four (A4) 340. The signal points each correspond to modulation symbols each composed of 4 bits, and Gray coding is performed for allocation as shown in FIG. 3 so that signal points, first and third bits of which are the same bits, exist in the same area. Therefore, in the illustrated constellation, first and second bit positions in each modulation symbol are higher-reliability bit positions (H), whereas third and fourth bit positions are lower-reliability bit positions (L).
Therefore, there is a need for a method of efficiently mapping data to each burst in the EDGE transmission system so as to enable a scheme of mapping important data and less important data to the higher-reliability bit positions and the lower-reliability bit positions, respectively.